This invention relates to a powder coating composition, and more particularly a powder coating composition that exhibits excellent resistance to degradation and has low gloss.
Powder coating compositions are now being used in a wide variety of painting techniques. They have become increasingly desirable, particularly in the automotive and truck industries, for various reasons including those related to ecology, health and safety. For example, powder coating compositions can reduce the amount of volatile solvents used as compared to liquid paints. After curing, typically using heat, only a very small amount, if any, of volatile solvent is given off to the environment.
Various powder coating compositions have been suggested. For example, a powder coating composition with excellent exterior durability can be prepared by reacting an acid group-containing acrylic polymer and a curing agent, triglycidyl isacyanurate (TGIC). Such coating compositions, however, often have poor stability and physical properties. Moreover, there is a concern about the environment impact of the use of TGIC.
U.S. Pat. No. 4,499,239 to Murakami et al. proposes a composition comprising 60 to 97 percent by weight of a linear polyester resin having an acid number of 15 to 200 mg KOH/g and 3 to 40 percent by weight of a glycidyl group-containing acrylic polymer, and optionally is modified with a monobasic acid such as methyl methacrylate. Powder coating compositions comprising a copolymer of glycidyl methacrylate, an ethylenically unsaturated compound, and a crosslinking agent formed in an anhydride of a dicerboxylic acid are proposed in U.S. Pat. Nos. 3,758,632, 3,781,379, 2,888,943 and 4,091,049 to Labana et al.
These compositions, however, tend to provide high gloss finishes. Powder coating compositions having a low gloss (matte) finish are becoming more desirable for use in automotive interiors, and for wheel rims, bumpers and the like. A common way to reduce gloss is to incorporate a filler such as talc or silica into the composition. It is believed that the gloss is lowered by the formation of minute surface irregularities on the coating. Another alternative is proposed in U.S. Pat. Nos. 5,436,311 and 5,525,370 to Hoebeke et al. which proposes a low gloss composition using a high percentage of methyl methacrylate in the composition. Other low gloss compositions are proposed in U.S. Pat. No. 4,242,253 to Yallourakis, and U.S. Pat. No. 5,491,202 to Umehara et al.
There, however, remains a need for a low gloss powder coating composition that has excellent resistance to degradation and eliminates the need for potentially hazardous modifiers or curing agents.